The time response of a tracking system of the type used in servo controlled fiber optic sensors can be limited by the type of dimensional measurement application to which the system can be applied. Typical systems are described in copending applications: Ser. No. 709,582 entitled "Precision Optical Displacement Measuring Instrument Using Servo Controlled Fiber Optic Sensor" now U.S. Pat. No. 4,674,882; Ser. No. 709,563 entitled "Improved Fiber Optic Sensor Probe"; and Ser. No. 709,560 entitled "Optical Configuration of Fiber Optic Sensor For Dynamic Response About The Optical Null", now abandoned, all of which were filed Mar. 8, 1985 and which we incorporated herein by reference. One of the primary limitations on such a system time response is often the response time of the circuitry used to produce a low noise position error signal proportional to the amplitude of the detected dither signal. Filtering of the detected dither signal is necessary to eliminate the harmonics of the dither frequency that are present in this signal. Filter circuitry that will attenuate dither signal harmonics generally has a slow response time that affects the time response of the axis motion.
One solution to this problem is to raise the values of both the dither frequency and the dither harmonic filter so that the filter response time is no longer a major factor in determining the axis motion response time. This solution is not practical for dither frequency values above a few hundred hertz because of the difficulty of designing the spring/mass system containing the lens assembly for the higher critical frequencies. A second solution to the problem is to eliminate the harmonic filter circuitry and develop a signal detection circuit that attenuates the harmonic signal components without affecting the system time response.